?????.....Your '65 is already 12V, neg ground. Nuthin' to change there.
Your current dash harness "should" have a radio power wire there already, so you can use it for juice, then just wire the speakers like the stereo's instructions say.
That's what I was thinking, but what do you use for the memory or illumination? Is the original radio power wire an ignition on wire, or does it have constant power? Just curious because the though has crossed my mind for my Scamp.